1. Field of the Invention
The present invention relates to a process for further treating a closure end made of sheet, particularly a folding end for beverage cans or the like. The tool for carrying out the process is also concerned.
2. Prior Art
Such an end is known from U.S. Pat. No. 3,441,170, where the radius of curvature itself is reduced in thickness from the inner side of the end thereby forming a bead ("coined bead") . In this way, kind of a joint is created between the inner leg of the groove and the central panel portion to restrict the bulging of the end to the central panel portion so as to reduce the tensile forces acting radially inwardly upon the core wall of the end. At this joint (coined bead), the central panel portion bulging more strongly under the increased internal pressure, is pivoted to the core wall such that the bulging does not affect or only affects slightly the latter as regards its vertical orientation.
EP 88 968 A1 discloses a similar measure in which, starting from the radially inner edge of the radius of curvature', the end sheet is externally deformed over an area of the radius of curvature' by pressing power, so that material of the end flows radially inwardly and outwardly from this radius of curvature region. The deformation region forms a flattening on the outer side of the radius of curvature', the major portion of the flattening being disposed in a plane perpendicular to the end axis or in a conical plane inclined outwardly and downwardly. This also serves for improving the resistance of the end to bulging. Owing to the flow of the material radially inwardly, compressive strain is applied to the outwardly bulged central panel portion thereby forming a free bulging ("free doming of central panel"), while the material flowing radially outwardly pivots permanently the inner leg of the groove Ushaped in cross-section from its original, inclined position into a position which is more cylindrical or parallel to the end axis ("permanent deflection of inner leg"). In the two known measures, the region deformed by coining ("coining") is simultaneously hardened by cold working ("work hardened") . Both prior art solutions strive to obtain an enlarged bulging of the central panel portion ("doming") . However, if a filled can provided with such a considerably bulged end is pasteurized, for example (it being placed upside down in this case), the resulting bulging will cause the can s t o tip and fall over.